1. Field of the Invention
This invention relates to a method for the recovery of gold from refractory ores. More specifically, this invention relates to the use of oxygen-enriched roasting to recover gold from refractory ores.
2. Background Art
In recent years the mining industry has become increasingly involved with developing methods for extracting gold from complex refractory ores. This trend is due to the increasing scarcity of free milling gold ore deposits. Gold ores are considered refractory when their gold content cannot be efficiently recovered by standard cyanidation techniques. Such ores are often refractory because of their excessive content of metallic sulfides or organic carbonaceous matter or both. It is not clearly understood why the sulfides and carbonaceous material cause such gold ores to be refractory but the phenomenon is well known in the mining industry.
The presence of carbonaceous matter, alone, can be efficiently dealt with by processes recently developed. U.S. Pat. No. 4,289,532 to Matson et al., hereby incorporated by reference, discloses a process that treats carbonaceous ores with an oxidation step utilizing chlorine and, in certain applications, oxygen to oxidize the carbonaceous matter prior to a cyanide leach step. Although this process was intended primarily to treat ores containing carbonaceous matter, the process has also been found effective in treating gold ores containing low levels of sulfidic compounds in addition to carbonaceous matter. The sulfidic compounds are oxidized by contact with chlorine. At higher ore sulfide concentrations, the oxidation reactions continue to occur, but excessively high quantities of chlorine are required and the process can become too costly. More drastic ore treatment techniques, such as autoclaving or roasting, can be necessary for ores containing high concentrations of refractory sulfidic compounds.
Autoclaving is defined within the industry as a process involving high pressure oxidation of ore in slurry form. Such a procedure is one alternative to chlorination in gold extraction technology. An oxygen-rich gas is typically used, rather than air, to increase the reaction rate. Autoclaving procedures in this industry suffer from temperature limitations, such as the boiling point of liquid phases, and mechanical design complications, such as the practical design and use of an agitated pressure vessel or the pressure let-down of an abrasive slurry. The operating costs associated with autoclaving tend to be higher than the costs for roasting, because of these problems.
The roasting of ores and ore concentrates to enhance the recovery of gold and other minerals has been practiced in many parts of the world. U.S. Pat. Nos. 2,878,102 to Sternfels, 3,150,960 to Hunter, and 3,174,848 to Bruce describe roasting processes for treating various mineral-containing sulfidic ores. The fluidized bed technique is widely used today for roasting various sulfide concentrates. Fluid bed roasting has excellent heat and mass transfer characteristics and provides a very efficient mechanism for the destruction of the refractory components in an ore while maintaining the efficient control of process temperature.
U.S. Pat. Nos. 3,995,987 to MacAskill, 4,076,796 to Yuki, 4,111,158 to Reh et al., and 4,389,380 to Parks disclose fluid bed roasting processes for treating various ores other than gold-containing ores. In particular the MacAskill process roasts primarily phosphate ores in a two stage reactor with air. This process is concerned with the elimination of carbonaceous matter from the ore and does not address the elimination of refractory sulfidic matter.
Although fluid bed roasting is known in the gold mining industry and has become the most favored roasting technique used to treat sulfidic gold ore concentrates, it has not been applied to unconcentrated gold ores. Standard fluid bed roasting is satisfactory for those gold ores that can be concentrated, however, many ores, for various reasons, cannot be concentrated easily or efficiently. Roasting of such ores has been attempted without success using kilns. This technique is an ancient procedure and does not have the advantages listed above for fluid bed roasting. The gold mining industry has suffered from the lack of a process for the fluid bed roasting of unconcentrated refractory gold ores.
The roasting of the ore concentrates which are refractory because of their high sulfide content is usually conducted under exothermic conditions because the proportion of sulfides in such concentrates tend to be high or between about 20 and 22 percent or more expressed as sulfur. Such exothermic roasting operations are autogenous with air oxidation, while the roasting of unconcentrated ores, with their low sulfide concentration are usually endothermic and will usually require external heat or fuel input. This distinction may have contributed to the delay in the application of fluid bed roasting processes to unconcentrated refractory gold ores.
The technology for roasting refractory gold ores is concerned almost exclusively with reducing the sulfide content. The added presence of carbonaceous matter in the refractory sulfidic ores is a factor that requires separate and serious consideration by the industry as less refractory ores become scarce. if the carbonaceous matter is not substantially eliminated from a complex refractory ore, the carbon adsorbs gold during subsequent cyanide leach operations, thereby reducing the amount of gold recovery. The presence of graphitic carbon or highly condensed organic carbon in endothermic ores further complicates the problem since this type of carbon is difficult to burn at temperatures below about 1350.degree. F. (732.degree. C.) in air and gold recovery tends to suffer if the ore is subjected to temperatures above about 1200.degree. F. (649.degree. C.). The reasons for impairment of gold recovery for ores subjected to elevated temperatures are not clearly understood but are believed to relate to gold encapsulation by glassy fluxes, surface tarnishing and other similar mechanisms. Thus, the presence of carbonaceous matter, particularly graphitic carbon, and highly condensed organic carbon makes the roasting process much more difficult. Temperature exposure must be sufficient to substantially destroy the graphitic carbon, as well as the other carbonaceous matter and the sulfides, yet the temperature exposure must be minimized to prevent impairment of gold recovery. Balancing these two parameters has been difficult, if not impossible to accomplish with the existing technology.
Thus a need exists in the gold mining industry for a roasting method which will efficiently treat unconcentrated refractory gold ores containing sulfidic matter or sulfidic matter combined with carbonaceous matter. A need also exists in the gold mining industry for a roasting method which minimizes used roasting processes while, at the same time operating in a temperature range low enough to minimize loss of gold recovery through encapsulation or other similar mechanisms.
It is, therefore, an object of this invention to provide an efficient method of treating refractory sulfidic gold ores so that the gold values of the ores can be economically recovered.
It is also an object of this invention to provide a method of treating refractory gold ores containing both sulfidic and carbonaceous matter so that the gold values of the ores can be economically recovered.
It is also an object of this invention to provide a method for roasting refractory sulfidic gold ores which minimizes environmental emission problems.
Another object of this invention is to provide a method of roasting refractory sulfidic gold ores which employs a low enough temperature range to avoid the formation of glassy fluxes or other similar mechanisms which impair gold recovery.